It is known that a water-ink emulsion with high water content may be formed in the pinking system during offset printing on areas of the plate that are free from image or carry little ink, and this emulsion causes streaky differences in ink management in these areas. Since the emulsion with high water content cannot be distributed any longer by the changing distributor roller because the distributor roller slips away over it, streaky fields with too much emulsified ink will form in the inking system during the operation of the press. This happens especially in film-type inking systems in which the ink splitting process of the ink is less intense because of the small gap that is always present between the duct roller and the film roller and therefore leads to toning of the printing plate and to undesired spraying of the ink.
Furthermore, it is known that especially in high-speed rotary offset presses, moistening agents split back into the inking system from the printing plate via the transfer rollers. The film roller as the last roller in the roller train, located at a short distance from the duct roller, cannot further split any moistening agent.
Circumferential water rings are formed on the hydrophilic film roller surface with flute-shaped recesses provided in the circumferential direction. The water rings building up compensate the distance from the adjacent duct roller, so that moistening agent reaches the ink duct via the duct roller. The homogeneous ink film on the duct roller is disturbed and the ink transfer up to the plate cylinder is adversely affected.
The moistening agent evaporation time, which becomes shorter in high-speed offset printing presses, has another adverse effect on the quantity of moistening agent in the ink duct.